Quaternary fans and terraces in the Khumbu Himal south of Mount Everest: their characteristics, age and formation

Large fans and terraces are frequent in the Khumbu Himal within the high Himalayan valleys south of Mt. Everest. These features are composed of massive matrixand clast-supported diamicts that were formed from both hyperconcentrated flows and coarse-grained debris flows. Cosmogenic radionuclide (CRN) exposure ages for boulders on fans and terraces indicate that periods of fan and terrace formation occurred at c. 16, c. 12, c. 8, c. 4 and c. 1.5 ka, and are broadly coincident with the timing of glaciation in the region. The dating precision is insufficient to resolve whether the surfaces formed before, during or after the correlated glacial advance. However, the sedimentology, and morphostratigraphic and geomorphological relationships suggest that fan and terrace sedimentation in this part of the Himalaya primarily occurs during glacier retreat and is thus paraglacial in origin. Furthermore, modern glacial-lake outburst floods and their associated deposits are common in the Khumbu Himal as the result of glacial retreat during historical times. We therefore suggest that Late Quaternary and Holocene fan and terrace formation and sediment transfer are probably linked to temporal changes in discharge and sediment load caused by glacier oscillations responding to climate change. The timing of major sedimentation events in this region can be correlated with fans and terraces in other parts of the Himalaya, suggesting that major sedimentation throughout the Himalaya is synchronous and tied to regional climatic oscillations. Bedrock incision rates calculated from strath terrace ages average c. 3.9 mm a , suggesting that the overall rate of incision is set by regional uplift. High Himalayan landscapes provide excellent natural laboratories to examine the interactions between tectonics, surface processes and climate change within an active continent–continent collision zone (Zeitler et al. 2001; Bishop et al. 2002). Furthermore, these regions contain excellent geomorphological and sedimentological records of glacial oscillations that provide important insights into the nature of Late Quaternary palaeoclimate change, notably the glacial and hydrological response to oscillations in the south Asian monsoon (Owen & Lehmkuhl 2000; Owen & Zhou 2002; Finkel et al. 2003). As a first step in examining the relationship between climate change and landscape evolution, we focus our study on landforms within the valleys of the Khumbu Himal region, south of Mount Everest (Figs 1 and 2). The region was chosen because it is relatively accessible and has well-established glacial chronologies, and contains abundant impressive fans and terraces that partially fill deep valleys downstream from modern glaciers (Muller 1958, 1980; Iwata 1976; Fushimi 1977, 1978; Williams 1983; Benn & Owen 1998; Owen et al. 1998; Aoki & Imamura, 1999; Richards et al. 2000; Finkel et al. 2003). (The fans may be described as ‘alluvial fans’, but we avoid the use of this term because it implies a genetic (fluvial) origin for the fanshaped landforms that are present within Himalayan valleys. These landforms predominantly comprise diamictons that may be of debris flow, hyperconcentrated flow and/or fluvial origin. We therefore use the non-genetic term ‘fan’ to describe these landforms.) These landforms (e.g. Fig. 1) allow us to begin to examine in detail the temporal and spatial relationships between climate change, glaciation, sediment transfer, and fan and terrace formation. In this paper, we report our investigations of these fans and terraces to provide evidence for the timing and rates of the erosional and depositional processes within this high Himalayan environment. We suggest temporal and genetic links between terrace and fan formation, and glacier oscillations. Our study of the fans and terraces in the Khumbu Himal and the related study on glacial stages by Finkel et al. (2003) are unique in the large number of samples dated (.100), number of landforms dated (14 fan or terrace surfaces, 15 moraines), great spatial distribution of the sampling (tens of kilometres), and integration with landform sedimentology. This represents the most extensive surface dating project ever attempted in the high Himalaya. Previous studies suggested that Quaternary fans within the Himalaya formed by rapid sedimentation during times of glacial retreat (Derbyshire & Owen 1990; Owen et al. 1995; Owen & Sharma 1998; Watanabe et al. 1998; Barnard et al. 2004a). These studies argued that fan formation occurred during times of glacial retreat and thus could be described as ‘paraglacial’. This follows the view of Ryder (1971a, b) and Church & Ryder (1972) that major episodes of deposition by non-glacial processes in proglacial environments are conditioned by glaciation. Moreover, Church & Slaymaker (1989) argued that enhanced sedimentation may persist for many millennia as landscapes adjust to non-glacial conditions. We aim to test these assertions by determining whether the Khumbu fans and terraces are paraglacial in origin and their formation is thus conditioned by climate, which in turn forces glaciation.